Portable electronic devices, such as portable phones or Portable Data Arrays (PDA), have been widely used, and had measures against static electricity put into. In order to design a circuit board easily, panel switches used as operation panels are provided with measures against static electricity.
Such panel switch includes movably contact assembly in which a movable contact made of elastic conductive metal processed to have a dome shape is held under an insulating film. This panel switch provides its operation with fine feeling, and provides electrically stable switching characteristics.
FIG. 7 is a sectional view of conventional movable contact assembly 5001 used for a panel switch disclosed in Japanese Patent Laid-Open Publication No. 11-232963. FIG. 8 is an exploded perspective view of movable contact assembly 5001. FIG. 9 is a sectional view of panel switch 5002 including movable contact assembly 5001.
Adhesive layer 2 is provided entirely on bottom surface 1A of flexible base sheet 1 made of insulating film, such as polyethylene terephthalate (PET). Movable contacts 3 electrically independent from each other are held on bottom surface 2A of adhesive layer 2. Movable contact 3 is made of elastic conductive metal processed to have a dome shape, and has concave surface 3A and convex surface 3B. Convex surface 3B of movable contact 3 is held at bottom surface 2A of adhesive layer 2. Conductive layer 5 is formed on upper surface 1B of base sheet 1. Conductive layer 5 is formed by printing and baking silver paste at 120° C. for 10 minutes. Separator 4 made of insulating film has surface 4A processed to be removable. Separator 4 is bonded onto portion 2C of adhesive layer on which movable contact 3 is not provided, thus positioning movable contact 3 between adhesive layer 2 and separator 4.
When movable contact assembly 5001 is used, as shown in FIG. 9, separator 4 is peeled off from adhesive layer 2, and portion 2C of bottom surface 2A of adhesive layer 2 is bonded onto surface 6A of circuit board 6. Circuit board 6 includes insulating board 6B and fixed contacts 7A and 7B provided on surface 6D of insulating board 6B. Surface 6A of circuit board 6 includes surface 6D of insulating board 6B and surfaces 7C and 7B of fixed contacts 7A and 7B. Portion 2C of bottom surface 2A of adhesive layer 2 is bonded onto fixed contact 7B and surface 6D. Fixed contact 7A faces concave surface 3A of movable contact 3. Outer periphery 7C of movable contact 3 is placed on fixed contact 7B. Movable contact 3 and fixed contacts 7A and 7B function as switch contacts.
An operation of panel switch 5002 will be described below. Force 5002F is applied from above upper surface 1B of base sheet 1 down toward movable contact 3 to reverse the dome shape of movable contact 3 elastically. Then, concave surface 3A of movable contact 3 contacts fixed contact 7A, thus connecting fixed contact 7A electrically with fixed contact 7B via movable contact 3.
Then, when force 5002F is removed, movable contact 3 returns to its original dome shape by its own elastic restoring force, and disconnects fixed contact 7A electrically from fixed contact 7B.
Base sheet 1 includes tongue portion 1C protruding therefrom. Conductive layer 5 has portion 5C formed on tongue portion 1C of base sheet 1. Tongue portion 1C is folded by 180 degrees at portion 1D and is bonded, so that portion 5C of conductive layer 5 contacts land 8 for grounding provided on insulating board 6B. Static electricity flowing from a human body to movable contact assembly 5001 via an operation button (not shown) flows into a ground via portion 5C of conductive layer 5 and land 8. This structure allows panel switch 5002 to have a small size.
Conductive layer 5 is formed by printing silver paste on base sheet 1 and baking the paste at a high temperature ranging from 120° C. to 150° C. for 10 minutes. This high temperature may cause base sheet 1 to deform, and increases energy cost to generate the high temperature.
The silver paste for forming conductive layer 5 may not be baked sufficiently in a temperature lower than the above temperature, and consequently, may cause conductive layer 5 to have cracks at portion 1D of base sheet 1 or to be peeled off from base sheet 1.